An igniter plug for a gas turbine engine conventionally comprises a tubular metal shell enclosing a ceramic insulator which, in turn, supports a metallic central electrode. A firing gap is formed between the tip of the central electrode and the periphery of the shell surrounding the electrode tip. The large difference between the thermal coefficients of expansion for ceramic material and for metal is a source of pernicious problems in such devices because of the high temperature ranges through which they must operate. The ceramic insulator cannot be secured at both ends of the shell since expansion of the shell would cause fracture of the insulator. When the insulator is secured to the shell at only one end, usually the end opposite the firing gap, expansion of the shell leads to undesirable changes in the configuration of the igniter at the firing gap.
To preserve the firing gap configuration in relatively long igniter plugs operating through wide temperature ranges it is current practice to form the electrical insulation thereof in two sections which are coaxially fitted together over a portion of their lengths. A shorter insulator section envelopes a portion of the end of a longer insulator section. The shorter insulator section is secured to the shell near the firing gap tip, while the longer insulator section is secured to the shell near the end opposite the firing gap, which opposite end is designed to mate with a connector for an ignition cable supplying electrical energy to the igniter. The disparate expansion rates of the shell and the insulator materials can then be accommodated without undesirable variation in the gap electrode configuration and without fracture of the insulator materials by the freedom of movement afforded by having the end portion of one insulator section telescope within the other insulator section. An example of an igniter having such telescoping insulator sections appears in U.S. Pat. No. 4,309,738, issued Jan. 5, 1982 to Mulkins et al. for Igniter Plug.
In the igniter plug described in the above referenced patent, fused glass seals are formed between the insulator and shell and between the insulator and central electrode near the connector end of the shell to prevent leakage of gases from the engine combustion chamber through the igniter plug. Although the insulator may be fitted relatively tightly within the shell when the igniter is assembled at ordinary ambient temperature, elevation of the igniter temperature to the high level encountered in use causes radial as well as lengthwise expansion of the shell. At operating temperature the major portion of the length of the longer section of the insulator is unsupported within the shell. Engine vibration transmitted through the unsupported length of the insulator then can cause cracking of the insulator near the supported end thereof or fracture of the glass seals formed in the vicinity of such supported end.
It is an object of the present invention to provide an igniter having a metal shell and a relatively long ceramic insulator with means effective at elevated temperature for supporting both ends of the insulator within the shell.
It is another object of the invention to provide an igniter having a metal shell and a ceramic insulator with means for supporting the insulator near both ends thereof so designed that thermal expansion of the shell will not exert damaging stress upon the insulator.
It is still another object of the invention to provide an igniter having a metal shell and a ceramic insulator with a metal supporting ring at the forward end of the insulator which is extruded into a tightly fitting conformal collar by the process of assembling the insulator within the shell.
Other objects and advantages of the invention will become apparent as an understanding thereof is gained through study of the following detailed description and accompanying drawings.